Photoconductive compounds which have so far been well known are inorganic materials including selenium, cadmium sulfide, zinc oxide, amorphous silicon and the like. These inorganic materials have an advantage of being endowed with satisfactory electrophotographic characteristics, namely very high photoconductivity and sufficient charge acceptance and insulation in the dark. On the other hand, they have also various disadvantages. More specifically, a selenium photosensitive material has disadvantages, e.g., in that it is high in production cost, lacks of flexibility and is weak in heat and mechanical impact; a cadmium sulfite photosensitive material has the problem of environmental pollution since cadmium known as a poisonous material is used as a raw material; a zinc oxide photosensitive material has difficulty in securing the image stability upon repeated use for a long term; and an amorphous silicon photosensitive material is extremely high in production cost and requires a special surface treatment for preventing its surface from deteriorating.
In recent years, electrophotographic photosensitive materials using various organic materials have been proposed with the intention of obviating the defects arising from those inorganic materials, and some of them have been put to practical use. For instance, the electrophotographic photosensitive material comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluorenone-9-one (U.S. Pat. No. 3,484,237), the electrophotographic photosensitive material comprising poly-N-vinylcarbazole sensitized with a pyrylium salt dye (JP-B-48-25658, the term "JP-B" as used herein means an "examined Japanese patent publication"), and the electrophotographic photosensitive material containing as a main component the eutectic crystal complex comprising a dye and a resin (JP-A-47-10735, the term "JP-A" as used herein means an "unexamined published Japanese patent application") are disclosed.
Further, recently, the electrophotographic photosensitive materials containing as a main component an organic pigment such as perylene pigments (e.g., U.S. Pat. No. 3,371,884), phthalocyanine pigments (e.g., U.S. Pat. Nos. 3,397,086, 4,666,802), azulenium salt pigments (e.g., JP-A-59-53850, JP-A-61-212542), squalium salt pigments (e.g., U.S. Pat. Nos. 4,396,610, 4,644,082) and polycyclic quinone pigments (e.g., JP-A-59-184348, JP-A-62-28738), and those containing as a main component an azo pigment as cited below have been studied. As a result, a great number of proposals have been made.
As for the azo pigments which have so far been studied, disazo pigments are disclosed in JP-A-53-133445, JP-A-59-78356, JP-A-59-128547, JP-A-61-57945, JP-A-61-17150, JP-A-62-251752, JP-A-62-273545, JP-B-63-18740, U.S. Pat. No. 4,504,559, JP-A-64-13555, JP-A-64-79753 and JP-B-2-4893, trisazo pigments are disclosed in JP-A-58-160358, JP-A-61-251865, JP-B-62-39626 and JP-B-63-10419, and tetrakisazo pigments are disclosed in JP-A61-182051 and JP-A-62-18565.
On the other hand, for example, presensitized plates using a positive working photosensitive material which contains a quinonediazide compound and a phenol resin as main components, and those using a negative working photosensitive material which contains an acrylic monomer or prepolymer as a main component, have been practically used as lithographic offset printing plates. Since these plates are all low in sensitivity, it is required of them to be in close contact with an original film, on which images have been recorded previously, in the exposure operation for producing therefrom the printing plates. In the meantime, owing to progress in both computer technology, including graphic processing and bulk data storage, and data communication technology, there has lately been put to practical use an electronic editing system in which a series of operations, involving input of original manuscript, amendment, compilation, layout and page make-up, are performed from first to last with a computer and the thus edited manuscript is transmitted immediately as the output to remote terminal plotters by a high-speed communications network or satellite communication. In particular, there is a great demand for the electronic editing system in the field of newspaper printing which requires the immediacy. Further, in a field such that original manuscripts are stored in the form of film and printing plates are reproduced from the films picked out among the stored ones in answer to requests, it can be expected that the development of bulk recording media such as an optical disc enables those original manuscripts to be stored as digital data in such recording media.
However, direct type printing plates, or printing plates produced directly from the output of a terminal plotter, rarely have practical utility. Even in the case that the electronic editing system is working, therefore, it is the present situation that a printing plate is produced by the method comprising the steps of recording the output on a silver salt photographic film, bringing the resulting film into contact with a presensitized plate and then performing an exposure operation. One reason for adoption of this method is that there have been difficulties in developing presensitized plates having sensitivities sufficient for the production of direct type printing plates within a practical time by the use of the light source of an output plotter (e.g., He--Ne laser, semiconductor laser).
As a photosensitive material having high photosensitivity enough to provide the direct type printing plate, an electrophotographic photosensitive material has been proposed.
Electrophotography utilized printing plate materials (original plates for printing) which have hitherto known include, e.g., the ZnO-resin dispersion offset printing plate materials disclosed, e.g., in JP-B-47-47610, JP-B-48-40002, JP-B-48-18325, JP-B-51-15766 and JP-B-51-25761. In using these materials as printing plates, they are wetted with a desensitizing solution (e.g., an acidic aqueous solution containing a ferrocyanate or ferricyanate) after the toner image formation by electrophotography, thereby desensitizing the non-image area. The offset printing plates which have undergone such a treatment as described above have an impression capacity of from 5,000 to 10,000 sheets. Those plates are unsuitable for more than 10,000 sheets of printing, and have a defect such that when the plate materials are designed so as to have a composition suitable for desensitization, they suffer from deterioration of electrostatic characteristics, and so the resulting plates cannot provide images of good quality. Further, there is a problem that a harmful cyan compound is used as a desensitizing solution.
In the organic photoconductive-resin printing plate materials disclosed, e.g., in JP-B-37-17162, JP-B-38-7758, JP-B-46-39405 and JP-B-52-2437, used are electrophotographic photosensitive materials in which a photoconductive insulation layer comprising, e.g., an oxazole or oxadiazole compound bound with a styrene-maleic anhydride copolymer is provided on a grained aluminum plate. After toner images are formed on these photosensitive materials by electrophotography, the non-image areas are removed by the dissolution in an alkaline organic solvent.
Further, the electrophotographically photosensitive printing plate material containing a hydrazone compound and barbituric or thiobarbituric acid is disclosed in JP-A-57-147656. Besides this material, there are known the dye-sensitized printing plates for electrophotographic process disclosed in, for example, JP-A-59-147335, JP-A-59-152456, JP-A-59-168462, JP-A-58-145495. However, such dye-sensitized printing plates failed in attaining sufficient sensitivity. Accordingly, there were held great expectations for the development of photoconductors having higher sensitivity.
As other means for realizing higher sensitivity, the photosensitive printing plate comprises a charge carrier generating compound dispersed in a resin binder, wherein a phthalocyanine compound, an azo compound or a condensed polycyclic quinone compound is used as the charge carrier generating compound, are known, e.g., in JP-A-55-161250, JP-A-56-146145 and JP-A-60-17751, yet they cannot be said to have sufficiently high sensitivity or satisfactory charge retention characteristics.
Although conventional organic electrophotographic photosensitive materials have appreciable improvements in mechanical characteristics and flexibility over the aforementioned inorganic ones, their sensitivities are still insufficient and some of them cause changes in electric characteristics by frequently repeated use. In other words, they do not always meet satisfactorily all the conditions required of an electrophotographic photosensitive material.
In addition, the sensitivities of the above-cited printing plates for electrophotographic process are generally insufficient for direct preparation of press plates without using any process film. In exceptional cases that the printing plates have high sensitivities, they are still insufficient for direct preparation of press plates because of some problems including their unsatisfactory charge retention characteristics.